Make sure you are using the dayOfWeek function, not the day function. (If you were real quick in viewing my response, you may have seen my post using the day function as I posted it initially with Day ( Today ) and then spotted my error, edited and reposted it just a few minutes later.)

Make sure that this is a calculation field. A date field with an auto-entered calculation will not work here.

If you have defined this as a field of type calculation, find it in Manage | Database Fields and double click it to bring up the specify calculatiosn dialog. In the Specify Calculations dialog, click the Storage Options button and you'll see a "do not store..." option.